This application is based on applications Nos. H11-273933, H11-273934, and H11-310084 filed in Japan, the contents of which are hereby incorporated by reference.
(1) Field of the Invention
The present invention relates to a sealed rectangular battery and a manufacturing method for the same. In particular, the invention relates to an improvement in the construction of the sealing cap and terminals.
(2) Related Art
A rapid increase in the use of small electronic appliances has been observed in recent years. Such appliances include mobile phones and personal digital assistants (PDAs), and need to be capable of relatively long periods of continuous operation. As a result, much research is being conducted into the development of batteries with high energy densities that can be used to power these appliances.
Small electronic appliances tend to be powered by sealed rectangular batteries. Compared to other batteries, rectangular batteries have the advantage of taking up little space. This makes it easy to enclose the battery within the internal space of a portable electronic appliance.
FIG. 8 shows a partial cross-section of the construction of a rectangular nickel-hydroxide battery as one example of a sealed rectangular battery. This nickel-hydroxide battery 10 is mainly composed of an external casing 20, a generator element 200, and a sealing cap 30. In this example, the nickel-hydroxide battery 10 is 35.5 mm high, 17.0 mm wide, and 6.1 mm thick.
The external casing 20 and sealing cap 30 are both press-formed from nickel-plated sheet steel. This means that the nickel-hydroxide battery 10 is enclosed by a metal casing. An electrode terminal 31 (the positive electrode in the illustrated example) is provided on the sealing cap 30, and is square-shaped with sides that are approximately 3.5 mm long. On the inside of the battery, a cylindrical member 313 (see FIG. 9B) is caulked to the positive electrode terminal 31 via the gasket 312. This part forms an airtight seal between the positive electrode terminal 31 and the sealing cap 30. The caulking referred to here is the process of partly deforming metallic members to fix them to one another.
The generator element 200 is composed of positive electrode plates 201, separators 202 and negative electrode plates 203 that are arranged in layers as shown in FIG. 8. This generator element 200 is impregnated with electrolyte and enclosed within the external casing 20. The negative electrode plates 203 are produced by forming a coat of a hydrogen-absorbing alloy (as an active material) on the surface of punching metal made of nickel-plated sheet steel. Conversely, the positive electrode plates 201 are produced by forming a coat of an active material, which has nickel hydroxide as its main constituent, on a surface of punching metal also made of nickel-plated sheet steel.
The positive electrode plates 201 are connected to the electrode terminal 31 via the tabs 2010 and the positive electrode collector 303.
The sealing cap 30 is arranged into an opening 15 of the external casing 20 so that an insulating plate 302 faces a spacer 21. Laser welding is then performed around the edges of the opening 15 and the sealing cap 30 to seal the inner space of the external casing 20.
FIGS. 9A to 9C show the sealing cap 30 in more detail. FIG. 9A shows the sealing cap 30 from above. FIG. 9B shows a partial cross-section of the sealing cap 30 taken along the line A-Axe2x80x2 shown in FIG. 9A. FIG. 9C shows the sealing cap 30 from below. As shown in FIG. 9B, a valving element 316 formed of elastic rubber is enclosed in the terminal cap 310. This valving element 316 normally presses against the periphery of the vent hole 315 to seal the battery airtight. However, when the pressure inside the battery rises to a predetermined value or above, the gas pressure inside the battery forces the valving element 316 upward so that gas that has accumulated within the battery can escape through the air outlets 311. As the gas escapes, the pressure inside the battery falls, so that the valving element 316 returns to its original position.
In addition to improvements in energy density, there are demands for miniaturization of sealed rectangular batteries (in particular for the development of slimmer batteries) and for reductions in internal resistance. Much research is currently being conducted towards these aims.
When developing a rectangular nickel-hydroxide battery, the construction of the positive electrode terminal with its comparatively large number of components places limitations on how slim the battery can be made. Such conventional designs make reductions in the thickness of the battery problematic. There is a further problem in that the complex construction of the positive electrode terminal raises the internal resistance of the battery.
When a battery is made slimmer, little distance is left between the positive electrode terminal and the edges of the sealing cap. These edges of the sealing cap are usually heat-sealed, such as by laser welding, to the edge that surrounds the opening in the external casing. When the resinous parts of the positive electrode terminal, such as the gasket, are located close to the welded parts, there is the risk of the heat generated during the sealing process causing deformation in the resinous parts. If, for example, the area surrounding the cylindrical member 313 of the electrode terminal 31 is exposed to heat, this will lead to deformation of the gasket 312 which can destroy the airtight seal for the battery. In this conventional structure, the electrode terminal 31 is electrically connected to the positive electrode plates 201 by the tab 2010 and the positive electrode collector 303. This makes the construction complex and lengthens the path taken the electrical current, thereby raising the internal resistance of the battery. These problems lead to decreases in battery performance.
As a result, there are demands for slimmer sealed rectangular batteries that do not suffer from decreases in battery performance.
The present invention has a first object of providing a sealed rectangular battery, and a manufacturing method for the same, that can be made slim without affecting the seal of the battery. The invention has a second object of providing a sealed rectangular battery, and a manufacturing method for the same, with reduced internal resistance.
The first object can be achieved by a sealed rectangular battery that has a battery case formed of an external casing and a sealing cap. The sealing cap encloses a generator element and is sealed by attaching the sealing cap using heat. The battery includes an electrode terminal with an opposite polarity to the battery case, and a safety valve. The safety valve is composed of a cap that covers a vent hole which is connected to an inside of the battery composed of (i) a valving element positioned so as to cover a vent hole that is connected to an inside of the sealed rectangular battery, and (ii) a cap provided so as to house the valving element and to become charged with a same polarity as the battery case. The electrode terminal and the safety valve are separately provided at different positions on the battery case.
With the stated construction, the electrode terminal can be made with a simpler construction than a conventional electrode terminal that is integrally formed with a safety valve. This means that the electrode terminal can be made smaller, so that the sealed rectangular battery can be made slimmer.
Since the electrode terminal can be miniaturized, when the sealing cap is attached to the external casing by the application of heat (such as by laser welding), adverse effects due to heat being conducted to the electrode terminal can be suppressed.
The safety valve can be charged with different polarity to the electrode terminal. When this is the case, the electrode terminal can be used as the positive electrode terminal and the safety valve can be used as the negative electrode terminal, for example.
The positive electrode terminal and the safety valve may be provided on a main surface of the sealing cap. With this construction, a battery pack can be produced just by connecting electrode terminals and safety valves on the sealing caps of sealed rectangular batteries without having to arrange adjacent batteries with opposite orientations. This improves the ease with which battery packs can be produced.
An outward cylindrical projection may also be formed on the outside of the battery case, with the shaft of the positive electrode terminal being threaded through an insulating gasket positioned in this projection.
When a positive electrode terminal is constructed in this way, the gasket is positioned far from the edge of the sealing cap. This avoids the conventional problem of deformation in the gasket due the effects of the heat used when attaching the sealing cap over the opening in the external casing. As a result, a sealed rectangular battery can be made slim without affecting the airtight seal.
When compared to a conventional battery where the electrode terminal is integrally formed with a safety valve, the battery of the present invention has a simpler construction connecting the generator element to the electrode terminal. When the electrode terminal is not integrally formed with a safety valve, fewer parts are required, which simplifies the connection and causes a corresponding reduction in the internal resistance of the battery.